Would you consider treating empirically for AVP deficiency in a patient who had pituitary surgery 6 months earlier and complaints of polyuria/polydipsia with dilute urine (without performing water deprivation studies)?  

In the appropriate clinical setting, like this, if you have documented inappropriately diluted urine, you can treat. The chance of resolution 6 months after surgery is very low. Always instruct the patient to have a daily polyuric phase to reduce the risk of hyponatremia.

I agree with these comments. The only thing that is a bit unclear about this question is if the symptoms are new in onset, or have been ongoing since the surgery. If the symptoms were new at 6 months and not present in the post op time frame, I would carefully question the patient about potential BP...